This invention relates to a transmission, in particular between a drive shaft and a wheel driving shaft of an engine, in particular of an automotive engine, such as a riding mower engine, whereby this transmission is at least of the belt-controlled variable-speed drive unit type combined with a clutch device, an engine, in particular an automotive engine, such as a riding mower engine, equipped with such a transmission as well as a process for operational control of such an engine.
Such transmissions are well known to those skilled in this art as, in particular, U.S. Pat. No. 6,755,759 illustrates. These transmissions generally consist of a variable-speed drive unit combined with a reduction clutch mechanism that is housed in a box. The clutch device is generally of the clutch-sleeve type. To date, such transmissions comprise two control elements that are actuated and that operate independently, namely a first control element that allows the passage of the clutch mechanism from the disengaged position to the engaged position, and a second control element that acts on the variable-speed drive unit, generally by controlling the speed variation by drawing together or separating the flanges of one of the pulleys of the variable-speed drive unit.
The problem of such a design is that the belt-controlled variable-speed drive unit can vary the rate of advance of the engine only within a determined range that goes from a minimum speed that is greater than zero to a maximum speed, whereby the minimum speed is based on the design characteristics of the variable-speed drive unit. It is therefore impossible to vary the rate of advance of the engine between zero and the minimum speed of the variable-speed drive unit, whereby this range of speed corresponds to the passage from the disengaged position to the engaged position of the clutch mechanism of the transmission, and whereby this clutch device is generally of the non-progressive type.
There are also transmissions that are applied to light automotive vehicles. Thus, the Patent EP-A-1,160,490 describes a belt-controlled transmission unit with a variable ratio in a continuous manner for vehicles with internal combustion engines, such as mini-vehicles. This unit comprises, housed in a box, a disk and jaw clutch and a belt-controlled variable-speed drive unit that is made up of two pulleys and a belt. A control element that is common to the clutch and to the variable-speed drive unit is provided. The operation of this unit is as follows. When there is no stress, the clutch is engaged, whereby the jaws are held against the disk via a spring. When the control element, which acts, on the one hand, on the spring to create the passage from the disengaged position to the engaged position of the clutch, and acts, on the other hand, on the drawing-together of the flanges of the variable-speed drive unit, is activated, it brings about, in a first step, by releasing the deformation of the spring of the clutch, the passage from the disengaged position to the engaged position of the clutch, then, in a second step, the drawing-together of the flanges in the direction of an increase of the speed. The drawback of such a control device is that it should, in a normally inactive position, apply force on the clutch device to keep the latter in a disengaged position, bringing about premature wear of the unit. Such a solution cannot be transferred to a riding mower engine where, for safety reasons, it is imperative that the transmission be disengaged in the case of failure, for example, of the control mechanism.
Patent EP-A-1,090,802 describes a transmission that comprises a disk clutch and a belt-controlled variable-speed drive unit controlled from a single control element that is made up of a lever that is moved by a cam. The lever acts on two thrust units that can move axially on the shaft that carries the clutch device and the pulley with movable flanges. Thus, in a first step, the lever allows an axial movement of the unit in terms of an engagement of the clutch device, then this lever acts via the thrust unit on the flanges of the pulley. This transmission comprises the same drawbacks as those mentioned in the Patent EP-A-1,160,490 since again, the control device in rest position acts for returning the clutch device into disengaged position.